Download image and update using NSURLSession

NBNet.m

@interface NBNet()
@property(nonatomic, strong) NSURLSession *imageSession;
@property(nonatomic, strong) NSOperationQueue *netOperationQueue;
@end 

@implementation NBNet

+ (id)sharedNBNet {
    static NBNet *sharedNBNet = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedNBNet = [[self alloc] init];
    });
    return sharedNBNet;
}

- (id)init
{
    self = [super init];
    if (self) {
        _netOperationQueue = [[NSOperationQueue alloc] init];
        _netOperationQueue.maxConcurrentOperationCount = 3;
        _netOperationQueue.name = @"Net Operations";

        NSURLSessionConfiguration *sessionImageConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
        sessionImageConfiguration.timeoutIntervalForResource = 6;
        sessionImageConfiguration.HTTPMaximumConnectionsPerHost = 2;
        sessionImageConfiguration.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
        
        _imageSession = [NSURLSession sessionWithConfiguration:sessionImageConfiguration delegate:nil delegateQueue:_netOperationQueue];
    }
    return self;
}

- (NSURLSessionTask *)imageWithURL:(NSURL *)url
                           success:(void (^)(UIImage *image))success
                           failure:(void (^)(NSError *error))failure {
    
    NSURLSessionTask *task = [_imageSession dataTaskWithURL:url completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        if (error)
            return failure(error);
        if (response)
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            //useless optimization as it seems to be decoded while UIImageView is displayed
            UIImage *image = [UIImage imageWithData:data];
            dispatch_async(dispatch_get_main_queue(), ^{
                if (image)
                    success(image);
            });
        });
    }];
    
    [task resume];
    return task;
}

UIImageView+NBNet.h

#import <UIKit/UIKit.h>
static char const * const taskKey = "TaskKey";

@interface UIImageView (NBNet)
- (void)setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;
- (void)cancelDownload;
@end

UIImageView+NBNet.m

#import "UIImageView+NBNet.h"
#import "NBNet.h"
#import <objc/runtime.h>

@interface UIImageView (NBNetProperty)
@property(nonatomic, strong) NSURLSessionTask *task;
@end

@implementation UIImageView (NBNet)

- (NSURLSessionTask *)task {
    return objc_getAssociatedObject(self,
                                    &taskKey);
}

- (void)setTask:(NSURLSessionTask *)newTask {
    objc_setAssociatedObject(self,
                             &taskKey,
                             newTask,
                             OBJC_ASSOCIATION_COPY);
}

- (void)setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder {
    self.image = placeholder;
    NBNet *net = [NBNet sharedNBNet];
    NSURLSessionTask *dTask = [net imageWithURL:url success:^(UIImage *image) {
        self.image = image;
    } failure:^(NSError *error) {
        DLOGV(4, net, error);
    }];
    self.task = dTask;
}

- (void)cancelDownload {
    [self.task cancel];
}

@end